The present invention relates to a method applicable to an electrophotographic copier or similar image recorder for controlling the density of an image in such a manner as to maintain it constant at all times.
In the above-described type of image recorder, a latent image electrostatically formed on an image carrier by a predetermined procedure is developed by a toner, i.e., colored fine particles fed from a developing device. The toner is usually charged to the opposite polarity to the latent image and electrostatically deposited on the latent image. To charge the toner to such a polarity, it may be combined with a carrier to constitute a two-component developer and agitated together with the carrier for frictional charging. While this kind of development using a two-component developer is capable of charging the toner sufficiently, the toner concentration sequentially decreases since only the toner is consumed during development. Therefore, the toner concentration of the developer, i.e., the density of an image to be developed by the toner has to be controlled to a predetermined value. This may be done by measuring the current toner concentration of the developer and, based on the measured toner concentration, controlling the amount of toner supply, i.e., the amount of toner to be fed to the carrier.
The toner concentration of the developer may be directly determined in terms of the weight or the permeability of the developer. Such direct measurement may be replaced with indirect measurement which uses a white reference pattern and a black reference pattern. Specifically, for the indirect measurement, latent images representative of a white and a black reference pattern are electrostatically formed on a photoconductive element and developed by a developer. The densities of the resulting toner images are measured by a photoelectric arrangement. More specifically, a photosensor or so-called P sensor is located in close proximity to the surface of the photoconductive element to sense the densities of the toner images of the reference patterns, so that a particular amount of toner supply is selected on the basis of the ratio of the sensed densities. This kind of scheme, therefore, determines a change in the density of each toner image of interest in terms of a change in the toner concentration of the developer, i.e., the mixture ratio of toner and carrier. An electrophotographic copier, for example, using such a method effects the measurement once every time ten copies are produced.
The conventional control method using a P sensor as stated above has the following problems left unsolved.
(1) Since the toner supply begins only after the toner concentration has lowered, the toner concentration sharply changes when documents of the kind consuming much toner are continuously copied, preventing the toner concentration from remaining constant.
(2) Since no consideration is given to the interval between the supply of toner and the resulting increase in toner concentration, the toner concentration is scattered over a broad range, i.e., the control accuracy is not satisfactory.
(3) Toner images representative of the reference patterns are formed once per ten copies without exception, as stated earlier. Hence, when a document of the kind consuming a relatively small amount of toner is copied a plurality of times, it is likely that a greater amount of toner is consumed by the toner images of the reference patterns than by the images of the document. On the other hand, when documents to be sequentially copied are of the kind consuming a great amount of toner, the conventional control method cannot accurately follow the change in the amount of toner.